1. Field of the Invention
The present invention relates to a method for reducing stress in a nickel-based alloy plating, and in particular, a method to plate a nickel-based alloy (such as Ni-W alloy or Ni-P alloy) having reduced stress by adding ceramic particles into the bath consisting of nickel/tungsten ion solution or nickel/phosphorous ion solution, in combination with the use of pulse current plating and the use of low temperature treatment.
2. Description of the Prior Art
Nickel alloys electroplated for engineering applications include nickel-tungsten, nickel-phosphorous, nickel-cobalt, nickel-manganese and so on. Among these nickel-based alloys, nickel-tungsten alloys particularly have improved resistance to sulfur embrittlement when heated. It has been found that after one hour, 600xc2x0 C. heat treatment, the hardness of nickel-tungsten alloy coating can reach 1000 Hv or even higher. In addition, coatings of nickel-tungsten show high resistance to corrosion. Accordingly, nickel-tungsten alloys are widely used in various industries such as the automobile industry, aerospace industry, food industry, printing applications, petrochemical/chemical industries, salt making industry, and medical industry. By way of example, the nickel-tungsten alloys may be applied on cylinders, pistons, rotors, compressors, rollers, molding parts, vessels, screens of centrifuges, or parts that are subject to strict corrosive environments.
Tungsten and phosphorous are typically used to increase the hardness and strength of nickel plating. It is known that as the concentration of tungsten in a nickel-tungsten alloy coating gets higher, the hardness and abrasion resistance thereof get better. However, on the other hand, as the concentration of tungsten in a nickel-tungsten alloy coating gets higher, the stress of the coating also raises. A thick nickel-tungsten coating having high stress results in cracking problems and poor adhesion ability. Therefore, researchers have strived to reduce the stress in nickel-tungsten alloy coatings. So far, there are generally two ways to reduce the stress of nickel-tungsten coatings. One approach is disclosed in both U.S. Pat. No. 5,853,556 and U.S. Pat. No. 6,045,682. They both use organic additives as a stress release agent. This has a drawback in that the organic compounds added in the bath will decompose due to the interaction of the electric field, thus generate a great deal of reaction byproducts, thereby deteriorating the coating. To remove the undesired reaction byproducts, active carbon or the like is added into the bath from time to time. This increases the loading of the bath management. In addition, the stress release agent may cause coating structure alterations and affecting the mechanical properties such as hardness of the coating. Another prior art method to reduce the stress of the nickel-tungsten coatings is so-called pulse-current electroplating. However, this pulse-current method is still not perfect.
The pulse-current electroplating is typically carried out by applied a pulse on the bath during plating process. The pulse output, which is somewhat similar to AC current, is preferably an asymmetric wave pulse. Particular parameters regarding the pulse-current electroplating process include On-Time (Ton), Off-Time (Toff), Duty cycle (=(Ton)/(Ton+Toff)), frequency (=1/(Ton+Toff)), and peak current density, and so on.
Furthermore, in order to increase the hardness of the nickel-tungsten coatings, the coatings are typically treated at 600xc2x0 C. for about one hour. This thermal treatment can bring out Ni4W, which is the main component that improves the material hardness up to 1000 Hv. However, the heat treatment also causes some reverse effects such as substrate softening or deformation, or, in a worse case, stripping of coating from the substrate.
Accordingly, there is a strong need for an improved plating method which is inexpensive and have good reliability and efficiency, and is capable of solving the above-mentioned problems.
The main objective of the claimed invention is to provide an improved plating method to reduce the stress of the nickel-based coatings such as nickel-tungsten coating and produce crack-free coatings without affecting the hardness of the coatings. According to the present invention, ceramic particles such as silicon carbide particles or tungsten carbide particles are added and evenly distributed in the nickel-tungsten coating.
Another objective of the claimed invention is to provide an improved plating method in combination with a low-temperature thermal treatment, thereby preventing substrate softening or deformation problems. The use of the low-temperature thermal treatment can slightly increase the hardness of the coating products. The use of the low-temperature thermal treatment can reduce the stress of the coatings since the hydrogen embrittlement resulting from exist of hydrogen in the coatings is eliminated.
The above object will be achieved by the method for reducing the stress in the nickel-based alloy plating provided in the present invention, which comprises the steps of: (a) adding ceramic particles into a plating bath containing soluble nickel salts; and (b) placing a substrate in the plating bath and thereafter carrying out a pulse-current electroplating in the plating bath.